The present invention is directed to a positive electrode active material for a magnesium secondary battery and a magnesium battery with a cathode based on the active material.
Lithium ion batteries have been in commercial use since 1991 and have been conventionally used as power sources for portable electronic devices. The technology associated with the construction and composition of the lithium ion battery (LIB) has been the subject of investigation and improvement and has matured to an extent where a state of art LIB battery is reported to have up to 700 Wh/L of energy density. However, even the most advanced LIB technology is not considered to be viable as a power source capable to meet the demands for a commercial electric vehicle (EV) in the future. For example, for a 300 mile range EV to have a power train equivalent to current conventional internal combustion engine vehicles, an EV battery pack having an energy density of approximately 2000 Wh/L is required. As this energy density is close to the theoretical limit of a lithium ion active material, technologies which can offer battery systems of higher energy density are under investigation.
Magnesium as a multivalent ion is an attractive alternate electrode material to lithium, which can potentially provide very high volumetric energy density. It has a highly negative standard potential of −2.375V vs. RHE, a low equivalent weight of 12.15 g/eq and a high melting point of 649° C. Compared to lithium, it is easy to handle, machine and dispose. Because of its greater relative abundance, it is lower in cost as a raw material than lithium and magnesium compounds are generally of lower toxicity than lithium compounds. All of these properties coupled with magnesium's reduced sensitivity to air and moisture compared to lithium, combine to make magnesium an attractive alternative to lithium as an anode material.
Production of a battery having an anode based on magnesium requires a cathode which can reversibly adsorb and desorb magnesium ions and an electrolyte system which will efficiently transport magnesium ions. Significant effort in each of these areas is ongoing in many research organizations throughout the world and active materials under investigation include sulfur in various forms, including elemental sulfur, materials known as Chevrel compounds of formula MgxMo6Tn, (wherein x is a number from 0 to 4, T is sulfur, selenium or tellurium, and n is 8) and various metal oxides such as MnO2 (alpha manganese dioxide stabilized by potassium), V2O5 and ion stabilized oxide or hollandiates of manganese, titanium or vanadium.
In this regard, cathodic active materials based on vanadium, such as V2O5 are extremely promising candidates for the Mg battery cathode, because vanadium is capable of multiple redox reactions between V5+/V4+/V3+ and V metal. Also, V5+ as a high valence state is quite stable, which means that it is easy to increase the operating voltage. Various research groups have reported efforts directed to utility of V2O5 as a positive electrode active material.
However, vanadium oxide suffers as an active material for insertion and deinsertion of magnesium ions because of the strong attraction of the Mg2+ ion for oxygen of the V2O5. This attraction leads to sluggish magnesium ion diffusion and hinders further magnesiation. Thus, low capacity and low rates are obtained with V2O5 without further modification.
Nanocrystalline V2O5 provides improved performance, however, it is conventionally known that nanocrystalline materials have low packing density and it is difficult to prepare a cathode having a desired high capacity and yet have sufficiently small dimensions to be useful in small scale appliance utility. Thus, the volumetric energy density of a cell employing nanocrystalline V2O5 would not be acceptable for commercial applications. Moreover, nanocrystalline materials promote electrolyte decomposition due to an extremely high surface area of the nanoparticles.
In an ongoing study of cathode active materials of high energy density for utility in a magnesium battery the present inventors have studied methods to mitigate the strong force of attraction of the magnesium ion for V2O5. Substitution of sulfur for oxygen in the active material in the form of an oxysulfide compound has been investigated.
Chen et al (U.S. 2013/0171502) describe a hybrid electrode assembly having a central current collector and on one side of the collector, a layer of a lithium ion intercalation material and on the other side a layer of an intercalation-free material such as a graphene. Conventional Li intercalation materials are listed in paragraphs [0072], [0104] and in claim 12. Included in the list are V2O5, V3O8, sulfur compounds and any mixture thereof.
Bedjaoui et al. (U.S. 2012/0070588) describe a method to package a lithium microbattery. In general description of a microbattery, titanium oxysulfide is described as a cathode active material.
Zhamu et al. (U.S. 2012/0064409) describes a lithium ion battery having electrodes containing nano-graphene-enhanced particulate materials. Conventional cathode active materials described include lithium vanadium oxide, doped lithium vanadium oxide, metal sulfides and combinations thereof. Explicit disclosure of a mixture of vanadium pentoxide and a sulfide glass forming agent is not made, nor is such a material suggested.
Gaillard et al. (U.S. Pat. No. 7,695,531) describe a photolithographic method to produce an electrolyte thin film for a lithium microbattery. In general description of a lithium microbattery components, titanium disulfide, titanium oxysulfide and vanadium oxide are listed as suitable cathode materials.
Gorchkov et al. (U.S. Pat. No. 6,916,579) describe cathode materials for a lithium ion or lithium metal battery which contains a crystalline vanadium oxide and a chalcogenide of sulfur, selenium or telurium. A mixture of vanadium pentoxide and a sulfide glass forming agent is not suggested.
Mukherjee et al. (U.S. Pat. No. 5,919,587) describe a composite cathode for an electrochemical cell which is constructed of an electroactive sulfur polymeric material and a transition metal chalcogenide. Other components such as silica, alumina and silicate may be present. Although, cells based on Group I and Group II metals are described generically, explicit disclosure of a magnesium electrochemical cell is not made. Vanadium pentoxide is disclosed as an example of the transition metal chalcogenide, however, a mixture of vanadium pentoxide and a sulfide glass forming agent is not made nor suggested. Phosphorous pentasulfide is not disclosed as a component of the cathode active material.
Abraham et al. (U.S. Pat. No. 4,934,922) describe a cathode active material being a transition metal oxysulfide, preferably molybdenum oxysulfide. Cells based on Group I and Group II metals are described generically, however, the focus is on lithium cells and explicit disclosure of a magnesium electrochemical cell is not made.
Ouvrard et al. (Journal of Power Sources, 54 (1995) 246-249) describes a vanadium oxysulfide compound of formula V2O3S.3H2O as an intercalation material for lithium ions. A lithium electrochemical cell having a positive electrode containing the vanadium oxysulfide is also described.
Aoyagi et al. (U.S. 2012/0164537) describes a positive electrode material for a magnesium battery. The cathodic material is a composite of vanadium oxide, phosphorous oxide, transition metal oxide and other elements such as alkali metals, sulfur and halogen. The composite is fused at specific temperatures and times to grow a mixed phase system containing vanadium oxide crystallites in an amorphous phosphorous oxide phase. In example I-28 a composition based on V2O5, P2O5, Fe2O3 and LiS is described. A mixture of vanadium pentoxide and phosphorous pentasulfide or any sulfide glass forming agent is not disclosed as a composition of a cathode active material.
Levi et al. (Chem. Mater. 2010, 22, 860-868) reviews the materials employed to date as positive electrode compositions and the problems associated with each. V2O5 aerogels are discussed; however, nowhere is a mixture of vanadium pentoxide and a sulfide glass forming agent disclosed or suggested.
Doe et al. (U.S. 2012/0219856) describe a series of spinel structure composites which serve as chalcogenides for a positive electrode for insertion and deinsertion of magnesium ion. Vanadyl phosphates are described among many others. However, this reference does not disclose or suggest a mixture of vanadium pentoxide and a sulfide glass forming agent.
Amatucci et al. (Journal of the Electrochemical Society, 148(8) A940-A950 (2001)) (listed in Invention Disclosure) describe a study of nanocrystalline V2O5 as an intercalation material for various cations. Although this reference indicates improvement in performance is necessary, disclosure or suggestion of mixture of vanadium pentoxide and a sulfide glass forming agent is not made.
Imamura et al. (Journal of the Electrochemical Society, 150(6) A753-A758 (2003)) report the synthesis and characterization of a V2O5 carbon composite as a positive electrode material for a magnesium battery, but does not disclose or suggest a mixture of vanadium pentoxide and a sulfide glass forming agent as a cathode active material.
Imamura et al. (Solid State Ionics, 161 (2003) 173-180) describes the performance of a V2O5 carbon composite as a positive electrode material for insertion and desertion of magnesium ion. Nowhere is a mixture of vanadium pentoxide and a sulfide glass forming agent disclosed or suggested.
None of the references cited disclose or suggest a mixture of vanadium pentoxide and a sulfide glass forming agent as a cathode active material for a magnesium battery.
Therefore, an object of the present invention is to provide a V2O5 based cathode active material which meets the requirements of a high energy magnesium battery and overcomes the deficiencies of the V2O5 forms conventionally known.
Another object of the present invention is to provide a positive electrode based on a modified V2O5 based material and a magnesium battery containing the positive electrode having significantly improved energy density and performance in comparison to known magnesium electrochemical devices.